1. Field of the Invention
The present invention relates to a thermosensitive recording material on which colored images are formed by heating. More particularly, the present invention relates to a thermosensitive recording material having a high thermosensitivity and being capable of forming thereon colored images resistant to fading and thus exhibiting a high degree of persistency during extended storage thereof.
The thermosensitive recording material of the present invention is capable of recording thereon colored images exhibiting an excellent resistance to moisture, heat, oily and fatty substances, and plasticizers, and thus has superior persistency when stored over a long period of time and therefore is useful as colored image-recording sheets, sheets for use in facsimiles, word processors, CRT image printers and cash dispensers, as passenger tickets, commuter passes, labels such as POS labels, cards such as prepaid cards, and as transit passes.
2. Description of the Related Arts
It is known that a conventional thermosensitive recording material comprises a supporting substrate, for example, a paper sheet, synthetic paper sheet, or plastic resin film and a thermosensitive colored image-forming layer formed on a surface of the supporting substrate and comprising a dye precursor, for example, an electron-donative leuco basic dye, a color-developing agent consisting of an electron-acceptive organic acid substance, for example, a phenolic compound, and a binder. When the thermosensitive colored image-forming layer is heated imagewise, colored images are recorded thereon by a reaction of the dye precursor with the color-developing agent.
This type of thermosensitive recording material is disclosed in Japanese Examined Patent Publication (Kokoku) Nos. 43-4,160 and 45-14,039 and Japanese Unexamined Patent Publication (Kokai) No. 48-27,736, and is widely employed in practice.
Namely, the thermosensitive recording material is advantageous in that colored images can be easily formed by heating alone, and the recording apparatus can be made compact and small in size, has a relatively low price, and can be easily maintained. Therefore, the thermosensitive recording material is appreciated as a useful information-recording material for recording outputs of printers used with, for example, computers, facsimile machines, automatic ticket-vending machines, scientific measurement recorders, and CRT medical measurement recorders.
Nevertheless, the conventional dye-forming type thermosensitive recording materials in which the thermosensitive colored image-forming layer comprises a conventional color-developing agent together with the dye precursor and the binder is disadvantageous in that the resultant colored images fade with the lapse of time, presumably because of a reversible reaction of the dye precursor with the color-developing agent. This fading of the colored images is accelerated by exposure to light, high temperatures, and high humidity and is specifically promoted by contact with an oily or fatty substance, for example, salad oil, or a plasticizer, to such an extent that the faded images cannot be recognized.
Many attempts have been made to retard or inhibit the fading of the colored images formed on a conventional thermosensitive colored image-forming layer containing a substantially colorless dye precursor comprising a lactone ring compound.
For example, Japanese Unexamined Patent Publication (Kokai) Nos. 60-78,782, 59-167,292, 59-114,096 and 59-93,387 disclose a thermosensitive colored image-forming layer containing a phenolic antioxidant.
Japanese Unexamined Patent Publication (Kokai) No. 56-146,794 discloses a protective layer formed from a hydrophobic polymeric compound emulsion on a thermosensitive colored image-forming layer.
Japanese Unexamined Patent Publication (Kokai) No. 58-199,189 discloses formation of both an intermediate layer and a top layer on a thermosensitive colored image-forming layer; the former being formed from a water-soluble polymeric compound solution or a hydrophobic polymeric compound emulsion and the latter being formed from a solvent-soluble hydrophobic polymer on the intermediate layer.
Japanese Unexamined Patent Publication (Kokai) No. 62-164,579 discloses a thermosensitive colored image-forming layer containing an epoxy compound in addition to a phenolic color-developing agent.
Japanese Unexamined Patent Publication (Kokai) No. 62-169,681 discloses metal salts of specific salicylic acid derivatives usable as a color-developing agent.
In the thermosensitive colored image-forming layer containing the phenolic antioxidant, the resultant colored images exhibit a higher resistance to heat and moisture to a certain extent compared to the colored images formed on a conventional colored image-forming layer free from the phenolic antioxidant, but the improvement effect of the phenolic antioxidant is not satisfactorily high. Also, the phenolic antioxidant does not have the capability to enhance the resistance of the colored images to the oily or fatty substances, for example, salad oil, and plasticizers, for example, dioctyl phthalate. The resistance of the colored images to oily or fatty substance or a plasticizer is determined in such a manner that the colored images are brought into contact with an oily or fatty substance, for example, a salad oil or a plasticizer, and left in contact therewith for a predetermined time, and then a retention of the color density of the tested colored images is measured in comparison with an initial color density thereof.
When the protective layer or the intermediate and top layers are formed on the thermosensitive colored image-forming layer, the resultant colored images exhibit a significantly enhanced persistency when the salad oil or the dioctyl phthalate is brought into contact with the colored image-forming surface of the recording material. Nevertheless, when the salad oil or the dioctyl phthalate is brought into contact with an edge face of the recording material, it penetrates the inside of the recording material and causes a complete fading of the colored images. Therefore, the provision of the protecting layer or the intermediate and top layer cannot completely eliminate the undesirable color-fading of the images.
The addition of the epoxy compound to the phenolic color developing agent, is not totally appreciated, because it takes a long time to stabilize the colored images formed on the colored image-forming layer after a heat-recording operation, and therefore, if salad oil or a plasticizer is brought into contact with the colored image-forming layer immediately after the heat-recording operation, the resultant colored images fade to a great extent.
The addition of the metal salts of the specific salicylic acid derivative to the colored image-forming layer effectively enhances the resistances of the colored image-forming layer to the oily or fatty substances and to the plasticizers. When the resultant thermosensitive recording sheet is subjected to a colored image-recording procedure and then to a heat resistance test, however, an undesirable color-development occurs on non-image-formed white portions of the recorded sheet. Also, the utilization of the specific salicylic acid derivative metal salts is disadvantageous in that this chemical has a complicated chemical structure and thus is expensive.
Generally, a thermosensitive recording material having a high persistency of colored images, which must have an additional surface layer, contains a special additive or use a special color-forming material, is disadvantageous in that the thermosensitivity is relatively low.